This invention relates to formulation of aluminum-containing ferritic stainless steel that is oxidizable to produce a protective surface layer characterized by multitudinous oxide whisker formations. More particularly, this invention relates to formulation of such steel from a contaminated melt and including a sulfur addition to neutralize magnesium impurity that would otherwise inhibit whisker formation.
Aluminum-containing ferritic stainless steel is particularly useful for high temperature applications, for example, as a substrate in an automotive catalytic converter. A typical steel comprises about 15 to 25 weight percent chromium, about 3 to 6 weight percent aluminum and the balance mainly iron. When exposed to oxygen at elevated temperatures, the steel forms a surface alumina layer that protects the underlying metal against further corrosion. The alloy may contain a minor addition of yttrium or rare earth metal, such as cerium or lanthanum, to promote oxide adhesion and thereby improve high temperature corrosion resistance. It is also known to add titanium, zirconium or hafnium to refine grain size, improve workability, counteract undesirably high carbon content and increase high temperature strength.
In contrast to the relatively smooth surface of the oxide layer that typically is formed on such stainless steel, it is known to oxidize the steel surface under conditions that produce a layer comprising oxide whiskers suitable to promote the bonding to applied coating. U.S. Pat. Nos. 4,331,631 and 4,318,828 describe oxidation treatments of foil formed of such iron-chromium-aluminum alloy that produce oxide whiskers that substantially cover the foil surface.
It has been found that the presence of magnesium impurity in amounts as little as 0.002 weight percent noticeably inhibits growth of the desired whiskers. U.S. Pat. No. 4,588,449 describes a treatment for growing oxide whiskers on magnesium-contaminated foil that comprises sustained heating to sublime the magnesium. While this treatment has been generally successful for reclaiming contaminated foil for whisker growth, it nevertheless requires a prolonged and expensive high temperature treatment.
Another typical contaminant in commercial aluminum-containing ferritic stainless steel foil is sulfur. In the absence of a suitable getter, most notably yttrium or rare earth elements, sulfur tends to reduce oxide adhesion and thereby diminish corrosion protection. It has now been found that, in the absence of yttrium or rare earth getter, sulfur reacts with the magnesium impurity, and further that the magnesium sulfide product does not inhibit formation of an adherent whisker layer. However, the sulfur impurity typically found in commercial steel is on the order of 0.003 weight percent and is insufficient to react with all magnesium, which is typically on the order of 0.01 weight percent. At the same time, it is necessary to protect the steel formulation from the unwanted effects of free sulfur.
Therefore, it is an object of this invention to provide a method for formulating an aluminum-containing stainless steel foil that is contaminated by magnesium impurity sufficient to inhibit oxide whisker formation, but which formulation renders the magnesium ineffective and thereby permits growth of oxide whiskers on the surface thereof.
More particularly, it is an object of this invention to provide a method for producing an aluminum-containing ferritic stainless steel from a melt contaminated with magnesium, which method includes adding sulfur in an amount stoichiometrically sufficient to react with all magnesium impurity and thereby to prevent free magnesium that would otherwise inhibit formation of oxide whiskers during subsequent oxidation of the steel. The treatment is necessarily carried out in the absence of any addition of yttrium or rare earth metals. In one aspect of this invention, the treatment comprises a further addition of titanium, zirconium or hafnium in amounts sufficient to react with any residual free sulfur that might otherwise reduce adherence of the oxide layer.
It is a further object of this invention to provide an aluminum-containing ferritic stainless steel containing magnesium impurity in an amount otherwise sufficient to inhibit oxide whisker growth, but further including a sulfur concentration adjusted to prevent free magnesium and thereby to permit whisker growth. In one aspect of this invention, the steel further includes a Group 4 addition to react with excess sulfur and thereby promote adhesion of a subsequently formed oxide layer thereon.
It is a still further object of this invention to provide a method for growing oxide whiskers on the surface of aluminum-containing ferritic stainless steel foil prepared from a magnesium contaminated melt, which includes adding sulfur to the melt in an amount effective to react with all magnesium and thereby eliminate the detrimental effects of free magnesium on whisker growth, and which optionally includes an addition of Group 4 metal to the melt to react with any excess sulfur and thereby promote adhesion of oxide whiskers subsequently grown on the foil.